I have a similar issue with my Ivy Bridge Thinkpad X230. Accoring to the terminal output, DXVK is crashing when attempting to call on unsupported Vulkan extensions in the Ivy Bridge driver. Though, in your case, the issue seems to be the AMD R600 driver which doesn't support Vulkan at all.
For example in my Thinkpad X240 which has a Haswell processor, the game works fine.
There's no references to DXVK in the Proton log at all and the game's requirements say it's an OpenGL 2.1 game. Are you getting confused by https://github.com/ValveSoftware/steam-for-linux/issues/5423?
Right you are! I have no idea, then.
I think I have a better hypothesis on what's going on here. On the three affected systems I have here, they all have 1366x768 (or lower) screens. I didn't notice it before, but the game engine wants to render to 1280x720, but it's getting cut off at the bottom. The combination of top/bottom DE bars + title bar + minor window chrome means that the game isn't getting the full height it wants and MATE is restricting the game's window to the visible area.
I was able to mash alt-tab until I could get to the options and set fullscreen, then the game started behaving.
It would be nice to know what the expected behavior is when the game is given a window smaller than it requested.
Tracking note: This game was removed from the whitelist today, and the game is now being unofficially tracked at #204, but the issue described here does not read like a game-specific issue, so I'm just dropping the Game Compatibility label so the issue report falls into the generic, unsorted issue set.
Compatibility Report
System Information
I confirm:
Symptoms
When running Into The Breach on Gentoo + Mate (software compositing enabled), the game renders fine on the test boxes here that have discrete vram, but stalls when on an integrated graphics system. I can get the game to render a couple more frames every time I alt-tab in and out of the game. If I disable software compositing in Mate's settings, the game no longer refreshes the rendered frame on alt-tab.
The rest of the DE and other applications remain responsive throughout testing and the game maxes out one cpu core. Nothing of real interest in the proton log spew.
I've been seeing this behavior since the initial release of Proton.
Reproduction
Run Into The Breach on Mate (1.18) and an integrated graphics system.